1. Field of the Invention
The present invention relates to an electrode of an electrode component, and more particularly, to an electrode component with an electrode layer formed of two or more different metal materials and a method for fabricating the same.
2. Description of the Related Art
A varistor is an electronic component mainly formed by zinc oxide powder and mixed with bismuth oxide, antimony oxide, manganese oxide and the like diffused to grain boundaries of zinc oxide. After the mixture is molded by a dry press process, organic binder is removed from the mixture and a ceramic resistor with nonlinear characteristics is generated from the molded mixture using a high-temperature sintering process.
The conductive electrode layer of a conventional varistor is usually formed by the silk-screen printing technique. During fabrication of the electrode layer, a ceramic chip with organic silver paste having a weight percent range of silver 60˜80% attached thereto is processed using an imbibition process in a temperature range of 600˜900° C. for the organic silver paste to form a desired electrode layer. The thickness of the electrode layer is normally maintained in a range of 6˜15 μm for soldering and product reliability. However, conventional silk-screen printing process has the following drawbacks and deficiencies.
1. Lots of toxic substances contained in the organic silver paste cause serious environmental pollution.
2. High production cost arises from the use of a great deal of precious silver material. To increase the surge-withstanding capability of the varistor, thick silver layer is inevitably adopted, and the thickness of the silver layer is oftentimes more than 15 μm.
The varistor with silver electrode fabricated using the conventional silk-screen printing process has the following shortcomings.
1. Low bonding strength due to the silver-ceramic incompatibility. The bonding strength is increased mainly through the glassy substance in the organic silver paste diffused to the grain boundaries of ceramic, such that the bonding strength between the silver electrode layer and the ceramic substrate is not satisfactory.
2. High-resistance ohmic contact.
3. Poor corrosion resistance of the silver electrode layer against lead-free solder. As the solid solubility of silver and tin is relatively high, solder can easily etch a silver layer at a high temperature. Nowadays, owing to the concern of environmental protection, products are manufactured using the lead-free soldering technique. To avoid pseudo soldering and melting silver, the 3Ag solder indicative of a Sn—Ag—Cu solder alloy with a higher silver content at a weight percentage of silver 3% is used for soldering and thus becomes a cost-down barrier of products. Meanwhile, because of the high mutual solubility of tin and silver in a lead-free solder, after products are powered on and operated for a long time, the silver electrode layer can be easily etched by the solder, such that the electrode has a reduced adhesion force and even becomes detached. Therefore, once the electrode becomes detached, transportation equipment, such as vehicles, using such type of varistor could be in a dangerous situation.
To lower production cost of the varistors, as disclosed in China Patent Application No. 201310177249.5, entitled “Base metal combination electrode of electronic ceramic element and preparation method therefor”, the drawback of the electrode of the varistor fabricated using a technique of hot-spraying multiple layers of base metal resides in that upon a high-voltage discharge current gives rise to high heat at metal electrode interfaces and the metal electrode interfaces could be easily separable, hindering durability and reliability of products.